Thermocouples Sensors

Pedro Castro

ECE 5320 Mechatronics

Assignment #1

Outline

What Is A Thermocouple Sensor?? Basic Working Principle Practical Thermocouple Construction Thermocouple Materials Standard Thermocouple Types Thermocouple Color Codes Characteristics Major Specifications Capabilities and Limitations Selecting A Temperature Sensor Comparisons

What Is A Thermocouple Sensor??

Thermocouples are among the easiest temperature sensors to use and obtain and are widely used in science and industry.

Thermocouples are the most common temperature sensing device. They can be made in very tough designs, they are very simple in operation and measure temperature at a point. Over different types they cover from -250C to +2500C.

Accurate temperature measurements can be made with thermocouples sensors at low cost with shop-built probes and ordinary low-level voltmeters.

Basic Working Principle

The principle of operation is on the Seebeck effect. A temperature gradient along a conductor creates an EMF. If two conductors of different materials are joined at one point, an EMF is created between the open ends which is dependent upon the temperature of the junction. As T1 increases, so does V. The EMF also depends on the temperature of the open ends T2.

The junction is placed in the process, the other end is in iced water at 0C. This is called the reference junction.

Practical Thermocouple Construction

A thermocouple construction consist of two conductors, welded together at the measuring point and insulated from each other long the length. It will usually have an outer protection sheath.

Thermocouple Materials

The three most common thermocouple materials for moderate temperatures are Iron-Constantan (Type J), Copper-Constantan (Type T), and Chromel-Alumel (Type K).

1-) The first named element of the pair is the positive element. 2-) The negative wire is color coded red.

All three types (J, K, and T) are available as insulated duplexed pairs from 0.001-inch diameter on up. For accuracy, and minimum system disturbance, the smaller the wire the better, but wire smaller than 0.003-inch diameter is very fragile.

Standard Thermocouple TypesIron-Constantan: Iron-Constantan (Type J, color coded white and red)

generates about 50 µV/°C (28 µV/°F). The Iron wire is magnetic. Junctions can be made by welding or soldering, using commonly available solders and fluxes.Iron-Constantan thermocouples can generate a galvanic EMF between the two wires and should not be used in applications where they might get wet.

Chromel Alumel: Chromel-Alumel (Type K, color coded yellow and red) generates about 40 µV/°C (22 µV/°F). The Alumel wire is magnetic. Junctions can be made by welding or soldering, but high temperature silver-solders and special fluxes must be used.Chromel-Alumel thermocouples generate electrical signals, while the wires are being bent, and should not be used on vibrating systems, unless strain relief loops can be provided.

Copper-Constantan: Copper-Constantan (Type T, color coded blue and red) generates about 40 µV/°C (22 µV/°F). Neither wire is magnetic. Junctions can be made by welding or soldering with commonly available solders and fluxes.Copper-Constantan thermocouples are very susceptible to conduction error, due to the high thermal conductivity of the copper, and should not be used unless long runs of wire (100 to 200 wire diameters) can be laid along an isotherm.

Thermocouple Color Codes

Thermocouple wiring is color coded by thermocouple types. Different countries utilize different color coding. Jacket coloring is sometimes a colored stripe instead of a solid color as shown.

Characteristics

Thermocouples have non-linear characteristics given by an approximating polynomial.

For example for type J (range 1, -210 to 760°C) the characteristic is given by

where V is in and T in °C

The output signal from a thermocouple is up to 50mV.

Major Specifications Type J: Iron / Constantan:Useful range of temperature

is -300F to 1200F. Maximum temperature 1600F. Possible problems: Oxidizes rapidly due to the iron wire. The use of the stainless steelmetal sheathed MgO style of construction has overcome some of this problem and is much preferred over the beaded bare wire style of thermocouple.

Type K: Chromel / Alumel Useful range of temperature is -300F to 1800F. Maximum temperature 2300F.

Type E: Chromel / ConstantanUseful range of temperature is -300F to 1800F. Maximum temperature 1000F.

Type T: Copper / ConstantanUseful range of temperature is -300F to 700F. Maximum temperature 700F.

Major Specifications……

Type R and S: Platinum / Platinum-Rhodium Useful range of temperature is 40F to 3000F. Maximum temperature 2300F.Both the R and S thermocouples are used for very high temperatures. These couples are relatively expensive compared to other thermocouples since they are made of platinum.These thermocouples must not touch the sheath if a metal sheath is used for construction. Normally a ceramic protective tube and ceramic beads are used for construction for both high temperature reasons and to prevent contamination of the noble metal.

Capabilities and Limitations

Capabilities:

- Wide Range

- Fast Response

- Passive

- Inexpensive

Limitations:

- CJC

- Non-Linear

Selecting A Temperature Sensor

      Choosing a temperature sensor can often be very straightforward, sometimes tricky, but always worth doing well. That's because these sensors, especially in science and engineering uses, can spell the difference between repeatable results and nonsense numbers. The name of the game in measurement is to measure with an amount of inaccuracy or uncertainty that is acceptable. So, the first thing you need to know is how well you need to know the value of the temperature numbers you expect to get. A simple series of questions, when answered, will usually get you started.

cont….

Selecting A Temperature SensorThe three things that we need to keep in mind when selecting temperature sensor:

1-) What is the desired temperature range, the tolerable limit to the error in measurement and the conditions under which the measurement is to be performed?

2-) Is it possible to touch the object and if so would the sensor or the temperature of the object be likely to be seriously affected by the contact?If the answer is yes, then a non-contact temperature sensor is needed.If no, then the answer probably lies with one of the other sensor types.

3-) If a contact sensor appears satisfactory, then questions revolve more around temperature measuring range, satisfying the conditions of use and meeting the acceptable error allowance.

Comparisons

Stability (Drift)Reasonable for limited lifetime Good Good Good

Repeatability Reasonable Good Good GoodHysteresis Excellent Good Good Good

Vibration Very Resistant Less Resistant Good Tolerant

Measurement Area Single Point Whole RT Element

Whole bead

(Small) Varies

DiameterSmall Sizes (to

0.25mm) Larger (3.0mm min)

Small (0.5mm

min) Varies

Linearity Not Linear Reasonably Linear Not LinearReasonably

Linear

Reference J unction Required Not RequiredNot

Required Not Required

Lead Wire Resistance No Problem Must be Considered No Problem Not RequiredContact Required Yes Yes Yes NoResponse Fast Slower Medium Fast

InfraredThermocoupleResistance

Thermometer (RTD) Thermistor